Excited singlet oxygen is probably an intermediate in many chemical processes initiated by light in the presence of oxygen. Photobiological effects such as the photosensitized inactivation of virus, may involve this intermediate. In this project singlet oxygen is generated directly with a Nd-YAG laser and its interactions with a number of molecules of biological interest are measured. Specific studies are on the determination of the chemical and physical quenching constants for singlet oxygen by the common amino acids, both free and incorporated within a protein structure. These results are of importance in understanding the mechanism of photosensitized oxidation of proteins. A second major study is the comparison between the self-sensitized photooxidation of bilirubin and biliverdin with the reaction with directly generated singlet oxygen. These reactions are probably the basis of the phototherapy treatment used to treat neonatal jaundice. BIBLIOGRAPHIC REFERENCES: "Low temperature absorption and fluorescence spectra and quantum yields of bilirubin " I.B.C. Matheson, G.J. Faini and John Lee, Photochem. Photobiol. 21, 135 (1975). "The quenching of singlet oxygen by amino acids and proteins", I.B.C. Matheson, R.D. Etheridge, N.R. Kratowich and John Lee, Photochem. Photobiol. 21, 165 (1975).